1. Field
The present invention relates to a laminate having high surface hardness and antistatic property, an optical film, a polarizing plate, and an image display device.
2. Description of the Related Art
In various image display devices, such as a liquid crystal display (LCD), a plasma display panel (PDP), an electroluminescence display (ELD), a cathode ray tube (CRT), and SED (Surface-Conduction Electron-Emitter Display), optical films used on the surfaces thereof are required to have antistatic property and high physical strength (e.g., abrasion resistance).
Importance of antistatic property has increased from both aspects of viewpoint of the use in final form of image display devices and viewpoint of the manufacture of image display devices. Dustproof property can be given by providing antistatic property.
As one technique adopted as a method to provide dustproof property, a method of forming what is called an antistatic layer by introducing particles having electric conductivity into an optical film to lower surface resistance is known (refer to JP-A-11-326602 (The term “JP-A” as used herein refers to an “unexamined published Japanese patent application”.)). However, particles having electric conductivity generally have a refractive index as high as 1.6 to 2.2 or so, so that the antistatic layer to be formed becomes a high refractive index layer. As a result, optical interference arises when refractive index difference takes place between the antistatic layer and the layer adjacent thereto (an optical film), or between the antistatic layer and the base material contiguous to the antistatic layer, which leads to a problem such that film thickness fluctuation and a minute defect of the antistatic layer are observed as unevenness of the optical film, and the improvement of the problem has been required.
With respect to the problem, an attempt is made to reduce interference unevenness by mixing the interface between the antistatic layer and the contiguous layer thereof. For example, an optical film formed by recoating an antistatic layer and a layer not having the function of antistatic property in the uncured state is disclosed in JP-A-2005-148444. However, since the interface is mixed by diffusing the particles in the antistatic layer, density of the particle lowers and contact probability of the particles is decreased even in such a film. As a result, electric conductivity lowers and there are cases where sufficient dustproof property cannot be obtained. There is a problem that loss in electric conductivity is great particularly when the strength of film is raised by increasing the amount of binder and the thickness of the antistatic layer is thin.
JP-A-2001-131485 discloses that antistatic property, transparency and surface hardness are compatible by arraying secondary particles having a particle size of 0.1 μm in a mesh-like state. However, the method of controlling aggregation of particles is not always sufficient. High aggregating property causes hard spot defect on the surface and cloudiness, in contrast with this, low aggregating property leads to insufficient conductivity. Further, conductive particles are generally high in refractive index, and the increase in refractive index of the film leads to a rise in reflectance and coloring.
Techniques of using an organic antistatic agent in a low refractive index layer containing a binder having alkali resistance are disclosed in JP-A-2005-316425 and JP-A-2007-293325. The use amount of the organic antistatic agent in the low refractive index layer is 0.3 to 5% by mass. The same patents describe that antistatic property can be obtained with the use of a small amount of organic antistatic component. Further, according to the same patents, the concentration of the antistatic agent may take the form of transition in the thickness direction in the low refractive index layer, and a conductive path may be formed such that the organic antistatic component exists in high concentration on the surface of the low refractive index layer. However, antistatic performance is not always sufficient in a cured film in which a small amount of organic antistatic component is localized in the vicinity of the surface, and persistence of it is not also sufficient.
The technique for providing excellent antistatic property without deteriorating various existing performances is an important problem common to the technical field of every sort and kind not limited to the field of optical film alone. Also from the point of manufacturing costs, the development of a technique capable of exhibiting excellent antistatic property with the addition of a small amount of antistatic agent has been eagerly desired.